The owners manual isn't what you think it is. It's really just an advertisement for the trailer, and tells you what options your trailer came with.
The owners manual you're looking for is more a package of individual owners manuals for each appliance in your camper: stove,
fridge,
furnace. But there's nothing which explains the entire
electrical system or how to "work" the camper.
If the outlets work, that means you're plugged in. Unless someone wired an inverter into your power center. Otherwise there shouldn't be any power to them.
Have you found the power center? Should be under one of the dinette seats. Metal box. It contains fuses and breakers. Check all the fuses. Check all the
light bulbs. That's the place to start, anyway. I guess...check to make sure you have a
battery and it has charge, too. Unfortunately with older campers you never know what's working, and what kind of crazy "modifications" previous owners have done.
Water heater has a pilot
light. You need to turn the knob to "pilot" and hold it there, then
light the burner with a lighter. Hold the knob for 10 or more seconds, then if the flame stays lit, turn the knob to "on". It should send out a flame thrower...There's a slight chance you have an electric ignition pilot light, and in that case I'm not sure. My 91
Bigfoot water heat has to be lit manually, but on my display center with
battery status, water tank status and water pump switch, there's a light that
lights up when the water pump is on. In the same place, it also has WH and GEN, for water heater and
generator. I don't have a
generator, and my water heater has no electronics, so I assume those
lights are for options I don't have. So maybe these trailers came with electric ignition water heaters back then as an option. I don't know.